Unlike a conventional single laser (serial) transmission link which transmits signals in series, a parallel optical interconnect system transmits signals in parallel. In operation, data signals in parallel form are input to signal processing and laser drive circuits at a transmitter. The circuits then control laser optical radiation emissions of the data signals. At a receiver, the optical signals are transformed back to electrical data signals via photo detectors and signal processing circuits. For the parallel optical interconnect system of the invention, a plurality of integrally formed vertical cavity surface emitting lasers (VCSELs) are used for signal transmission from the transmitter.
Despite its success in achieving higher data transmission speed compared to conventional single or serial transmission link, a parallel optical interconnect system with VCSELs inevitably encounters problems in performance control and reliability, some of which arise from the nature of parallel signal processing.
One major problem stems from VCSEL performance. The output optical power of the VCSELs fluctuates due to changing environment such as temperature variation, aging behavior of the VCSELs, or circuit property drift in the laser drive circuitry. In the conventional single or serial transmission link, the data output from the laser is coded to be dc balanced. The dc balancing technique allows one to place a photodetector monitor at one face of the laser that is not coupled into the optical fiber. The photodetector receives a proportional fraction of the laser light that is emitted from the laser, and delivers a feedback signal to the laser drive circuit to correct the laser output optical power fluctuation. However, this method is inefficient and troublesome for a parallel optical interconnect system. It would require a photodetector and a feedback circuit for each and every one of the VCSELs. In addition, the dc balance technique is incompatible with the use of uncoded dc signals as in present parallel optical interconnect systems.
Recently, the inventor has developed a new method in which the monitoring photodetector is integrated into VCSELs so as to provide for adjustment of a laser output optical power in a parallel optical interconnect system., see U.S. patent application Ser. No. 08/217,531, which is incorporated herein in its entirety by reference. However, there still remains a significant need for a method and apparatus to monitor and automatically compensate for the laser output optical power fluctuation, in order to reliably utilize the parallel optical interconnect system.
Furthermore, the laser output optical power fluctuation at the transmitter affects the data signal retrieval at the receiver. Thus, there also exists a need to properly retrieve data signals despite any signal fluctuation.
Another kind of problem comes from the nature of parallel processing. Unlike a conventional single (serial) transmission link which typically uses ac coupled receivers, each having a clock recovery circuit, a parallel optical interconnect system has skew, i.e., the signals on a set of parallel interconnects do not all arrive at the receiver simultaneously. The clock signal is transmitted on a separate line in parallel with the data. Skew may be caused by the same factors that lead to output optical power fluctuation of a VCSEL. For example, since the signals being transmitted in the parallel optical interconnect system are uncoded and not dc balanced, some VCSELs transmit more high level (1 level) signals than others during transmission. This almost ensures that these VCSELs have higher temperatures during operation. The temperature difference may be quite significant from one end of the VCSEL array to the other. This laser substrate temperature difference is just one source of skew problems.
A weak clock signal conceivably is another source of skew in the parallel optical interconnect system. Because the clock signal is transmitted with data and this clock signal is used to extract the data signals, there is likely to be skew if there is uncertainty about the exact location in time of the clock transition.
Since skew may cause serious distortion of data signals during transmission, thus offsetting the advantage of using a parallel system, it is essential for a parallel optical interconnect system to have minimal skew.